1. Field of the Invention
The present invention relates to an optical passive device, and more particularly, to an amplitude mask and an apparatus for manufacturing a long period grating filter using the same.
2. Description of the Related Art
Recently, with the development of optical communications, the long period grating filter is arousing much interest as an optical passive device. The long period grating filter is a device for coupling light propagating in the core of an optical fiber to a cladding mode. The long period grating filter is manufactured by periodically changing the refractive index of the core of an optical fiber sensitive to ultraviolet rays. Namely, the refractive index increases in the part exposed to light and does not change in the part not exposed to light. Accordingly, the refractive index periodically changes. The following Equation 1 must be satisfied in order to couple light in a core mode to the cladding mode. ##EQU1##
wherein, .beta..sub.co, .beta..sub.cl.sup.n, and respectively denote a light propagation constant of the core mode, a light propagation constant of an n degree cladding mode, and a grating period.
In Equation 1, when 2.pi.n/.lambda. (n is a refractive index, and .lambda. is a wavelength) is substituted for .beta., n.sub.co -n.sub.cl =N is obtained. Here, n.sub.co is a refractive index of the core, and n.sub.cl is a refractive index of the cladding in an optical fiber. Therefore, in order to change light having a particular wavelength into a cladding mode, the period and the refractive index difference n.sub.co -n.sub.cl are to be defined. The refractive index difference can be obtained by appropriately exposing the ultraviolet sensitive optical fiber to ultraviolet laser light.
The apparatus for manufacturing the long period grating filter by irradiating with an ultraviolet laser light will be described.
FIG. 1 schematically shows a conventional apparatus for manufacturing a long period grating filter.
In detail, the conventional apparatus for manufacturing the long period grating filter includes a high-output excimer laser light source 1 for emitting UV laser light, a mirror 3 for changing the path 2 of the laser light emitted from the excimer laser light source 1, a lens 5 for controlling the focus of the laser light changed in direction by the mirror 3, a silica mask 7 for selectively transmitting the laser light which has passed through the lens, and an optical fiber 9 for forming the long period grating in the core by irradiation with the laser light which has passed through the silica mask 7.
In a process of manufacturing the long period grating filter using the manufacturing apparatus shown in FIG. 1, the laser light irradiates the optical fiber 9 which contacts the silica mask 7 through the lens 5. At this time, the laser light irradiates the optical fiber 9, thus forming a long period grating having a different refractive index. It is possible to obtain a desired long period grating filter by transmitting light through the optical fiber 9 using the light source 11 and detecting light with a detector 13.
In the above-mentioned apparatus for manufacturing the long period grating filter, the silica mask 7 is formed using a chrome pattern obtained by coating a silica substrate with chrome and patterning the chrome. Therefore, the laser light is selectively transmitted by the chrome pattern. However, since the damage threshold of the chrome pattern is low, i.e., 100 mJ/cm.sup.2, it is not possible to effectively use the high-output excimer laser light. Further, since the silica mask is manufactured by forming the chrome pattern on the silica substrate, the silica mask has only one period.